FIG. 11 is a block diagram of a conventional step-down type DC/DC converter used in a handy phone. As shown in FIG. 11, a series circuit of first switch circuit 100, inductor 102, and smoothing capacitor 104 is connected between input terminal 111 and output terminal 112. Output voltage 120 from the smoothing capacitor 104 and control voltage 121 from output voltage control terminal 113 are put into comparator 105. Output signal from this comparator 105 and output signal from oscillation circuit 106 are put into switch condition determining circuit 107. Output voltage 120 and switch current detection signal 122 from the first switch circuit 100 are put into control operation determining circuit 110, and the control operation determining circuit 110 judges the load, and feeds its output signal 123 into the oscillation circuit 106 and switch control circuit 108. The switch control circuit 108 receives the output of the switch condition determining circuit 107 and output signal 123 of the control operation determining circuit 110. Thus, the switch control circuit 108 supplies a signal for controlling the first switch circuit 100 to the first switch circuit 100. Feedback is composed in this manner.
Such prior art is disclosed, for example, in Japanese Laid-open Patent No. H7-322608.
In this conventional step-down type DC/DC converter, however, transient response time is long when lowering the voltage in step-down process of output voltage, and the output voltage cannot be lowered in a short time. As a result, power loss occurs during transient response time, and the battery voltage is decreased, and it is hard to extend the call time.
The conventional step-down DC/DC converter operates only in step-down process. That is, the conventional DC/DC converter cannot be used in step-up operation. Further, the conventional DC/DC converter is slow in response speed in transient response depending on the load situation, and wasteful power consumption occurs.